Hybrid solar cells combining organic polymers and inorganic semiconductors are extensively investigated recently due to relatively inexpensive cost and simple fabrication processes. In this work, we demonstrate organic/inorganic hybrid heterojunction solar cells based on gallium arsenide (GaAs) substrate and conjugated polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS). First we performed a one-dimensional device simulation based on a self-consistent Poisson and drift-diffusion solver to survey the band alignment between the conductive polymer and GaAs materials and achieve a practical device design. Second, for device fabrication, we prepare a cleaned one-side-polished planar GaAs wafer, followed by thermal evaporation of back-side metal using either Aluminum or titanium/gold as the cathode. Next, PEDOT:PSS is spun-cast onto the wafer and annealed at 115°C for 10 minutes. To improve carrier conduction, we use a self-assembled polystyrene (PS) nanosphere lithography technique to form the sacrificial mask layer, and perform anisotropic metal-assisted chemical etching on GaAs substrates. Various nanostructures such as nanowires or nanorods allow the conformal p-n heterojunction formation at the interface of organic/inorganic semiconductors, which can be beneficial for both light absorption and carrier collection. The optical and electrical characteristics such as reflectance, current voltage, and external quantum efficiency are measured. Currently, we achieve a 3.2% power conversion efficiency with an open-circuit voltage of 0.565 V, short-circuit current of 8.95 mA/cm2, and a fill factor of 63.29% under a simulated AM1.5G illumination for planar substrates. Device fabrication with GaAs nanowires is still in process and more data will be presented.